Project 4. Abstract Theory and modeling of the orofacial active sensation (Golomb as lead; Freund; Kleinfeld) This Research Project addresses the dynamics of the sensorimotor brainstem circuits for active sensing. Our initial focus is on the vibrissae system, given the relative plethora of information on this aspect of orofacial behavior. The specific elements in our model include the vibrissae, the vibrissa motor plant, and sensory and motor neuronal ganglia and nuclei. We will use theoretical and computational techniques to describe the activity of several Components and, of great generality to perception, how active feedback alters the nature of the sensory input as a consequence ofs active sensing. First we will analyze the interaction of the vibrissae and the motor plant, specifically how contact effects the motor plant and the forces and torques on the vibrissae. We then quantify the transformation of mechanical signals in whisker follicle to neuronal spiking of trigeminal ganglion neurons during vibrissa movement and touch with object. At the motor brainstem level, we explore the generation and synchronization of firing patterns of motoneurons and pre-motoneurons, and their reset by the pre-Btzinger complex. Finally, we will construct models of larger brainstem loops and explore general issues of how sensory signals control motor activity during behavior. Our models will be constrained by connectivity data from Research Project 1 and physiological data about the activation and operation of orofacial exploratory circuitry from Research Project 2 and 3, and their predictions will be tested in Research Project 2 and 3.